Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4455d755eb61
Author: root <root@localhost.localdomain>
Desc: Bug 923821 - Repair and re-enable test_killall.py TestKillAll.test_kill_all. r=zcampbell r=dhunt